Radiation transport of heliospheric Lyman-alpha from combined Cassini and Voyager data sets

Radiation transport of heliospheric Lyman-alpha from combined Cassini and Voyager data sets Wayne Pryor1, Pradip Gangopadhyay2, Bill Sandel3, Terry Forrester3, Eric Quemerais4, Eberhard Moebius5, Larry Esposito6, Ian Stewart6, Bill McClintock6, Alain Jouchoux6, Tom Woods6, Joshua Colwell6, Vladimir Izmodenov7, Kent Tobiska8, Joseph Ajello9, Candice Hansen9, Klaus Scherer10, Maciej Bzowski11 and Priscilla Frisch12 Central Arizona College University of Southern California Lunar and Planetary Laboratory/University of Arizona Service de Aeronomie University of New Hampshire Laboratory for Atmospheric and Space Physics/University of Colorado Lomonosov Moscow State University Space Environment Technologies Jet Propulsion Laboratory Institute for Theoretische Physik IV, Ruhr-Universitat Bochum 10 Space Research Centre, Polish Academy of Sciences 11 University of Chicago 12 wayne.pryor@centralaz.edu To be submitted to Astronomy and Astrophysics special section May 2007 Abstract Heliospheric neutral hydrogen scatters solar Lyman-alpha radiation from the sun with “27-day” intensity modulations observed near Earth due to the Sun’s rotation combined with Earth's orbital motion These waves are increasingly damped at larger distances from the Sun due to multiple scattering in the heliosphere Data from Pioneer, Voyager and Cassini provide several examples of this damping process Shemansky et al., 1984 found the Lyman-alpha intensity waves in 1982 were weakly reduced in amplitude at Voyager (at 10 A.U.) and strongly damped at Pioneer 10 (at 30 AU), leading to termination shock hydrogen density estimates of ~0.16 and 0.11 cm-3 respectively Quemerais et al., 1996 studied Voyager and Lyman-alpha data from 1981-1993 and found the solar waves were damped in amplitude by a factor of ~0.4 at the end of this period when the spacecraft were 44 and 56 A.U from the Sun, implying a hydrogen density at the termination shock of 0.15 +/- 0.10 cm -3 This paper presents additional Voyager data from 2003-2004 obtained at a distance of 88.8-92.6 A.U from the Sun with waves damped by a factor of ~0.21 Simultaneous Cassini data obtained downwind near Saturn at ~10 A.U at times show undamped "27-day" waves in good agreement with the single-scattering models of Pryor et al., 1992 We conclude that multiple scattering is definitely occurring in the outer heliosphere The observed degree of damping is interpreted in terms of Monte Carlo multiple-scattering calculations (e.g., Keller et al., 1981) applied to heliospheric hydrogen two-shock density models provided by Izmodenov and discussed in Gangopadhyay et al., 2006 The two models lead to an inferred heliospheric neutral H density at the termination shock near 0.085 cm-3 independent of ultraviolet instrument calibrations, although the result is somewhat model-dependent This work generally agrees with earlier discussions in Quemerais et al., 1996 showing the importance of multiple scattering but is based on Voyager data obtained at larger distances from the Sun (with larger damping) simultaneously with Cassini data obtained closer to the Sun Introduction Interplanetary Lyman-alpha radiation, first detected by the Orbiting Geophysical Observatory OGO-5 (Bertaux and Blamont, 1971; Thomas and Krassa, 1971) is seen in all directions, and is the brightest UV emission from interplanetary gas As discussed by many authors, and reviewed in Thomas 1978, interplanetary Lyman-alpha is produced by resonance scattering of solar Lyman-alpha by interstellar hydrogen gas approaching the Sun from the “upwind” direction Hydrogen loss processes near the Sun (primarily charge-exchange with solar wind protons, and a smaller contribution from solar EUV photoionization) lead to a cavity depleted in slow neutral H that can scatter Lyman-alpha The heliospheric Lyman-alpha intensity seen in any direction varies as the Sun rotates, because the Sun’s UV emissions are enhanced in localized active regions, generally at low solar latitudes This modulation in the heliospheric Lyman-alpha intensity can be estimated from a hydrogen hot model that calculates hydrogen densities (e.g., Thomas 1978) coupled to a single-scattering radiative transfer calculation that integrates model emission rates along a given line of sight that can then be compared to data The emissions in each direction from the Sun are estimated using solar Lymanalpha values measured by spacecraft near Earth such as UARS (Upper Atmosphere Research Satellite) SOLSTICE (Solar-Stellar Irradiance Comparison Experiment) or TIMED (Thermosphere Ionosphere Mesophere Energetics and Dynamics) SEE (Solar EUV Experiment) Hot models neglect outer heliospheric effects on the hydrogen population, but are still frequently used to describe the hydrogen population inside the termination shock As first demonstrated by Shemansky et al., 1984, the Lyman-alpha modulations seen in heliospheric data are reduced in the outer heliosphere compared to the single-scattering models, providing evidence that multiple scattering is significant in forming the observed emission Multiple scattering acts to reduce the flux differences observed in the heliosphere by increasing the range of angles over which a localized solar bright spot illuminates the heliosphere (Quemerais et al, 1996) Light travel-time effects from multiple scattering could also act to reduce the 27-day brightness modulations, but are a minor effect, since most of the scatterings of interest occur within light-day of the Sun (1 A.U corresponds to light-minutes) In this paper, we compare recent Voyager and Cassini data sets to a singlescattering model, demonstrate the presence of damping in the Voyager data, and then assess the resulting damping factors in terms of multiple scattering models in order to derive an estimate of the interplanetary H density at large distances from the Sun (but inside the postulated H wall outside the recently detected (Stone et al., 2005) termination shock seen at 94 A.U from the Sun The derived hydrogen density estimates will not refer to the local interstellar medium value, but rather the "processed value" after chargeexchange filtration through the outer heliospheric shock structures has reduced the interstellar neutral hydrogen density to a lower density level near the termination shock These estimates will be compared to other H density estimates near the termination shock from hydrogen pickup ion measurements and solar wind slowdown measurements described in companion papers in this issue (Bzowski et al., 2007; Richardson et al., 2007) Observations We will mention interplanetary Lyman-alpha data from five spacecraft instruments: the Pioneer UV photometers (Judge and Carlson, 1974; Carlson and Judge, 1974) mounted on Pioneer 10, leaving the solar system in the downwind direction, and on Pioneer 11, leaving upwind; the Voyager Ultraviolet Spectrometers (UVS, Broadfoot et al., 1977), mounted on the Voyager and spacecraft and now leaving the solar system in the upwind direction; and the Cassini Ultraviolet Imaging Spectrograph (UVIS, Esposito et al., 2004) on the Cassini orbiter mission to Saturn, downwind at Saturn arrival in 2004 Specific examples to be discussed are taken from different solar cycles Periods near solar maximum are best when large active regions on the Sun are most likely to be present, creating 27-day waves of significant amplitude The first example, presented by Shemansky et al., 1984, was Voyager and Pioneer 10 interplanetary Lyman-alpha modulation data from 1982 used to infer hot model hydrogen density at large distances from the sun Voyager interplanetary data obtained at ~12 AU out from the Sun in 1982 show modulations almost as large as the modulations in the Solar Mesosphere Explorer (SME) solar Lyman-alpha variation Shemansky et al., 1984 interpreted the Voyager data as indicating the outer heliospheric neutral hydrogen density = 0.16-0.17 cm-3 based on multiple scattering calculations for a hot hydrogen model presented in Keller et al., 1981 Pioneer 10 data from 30 AU out from the Sun from the same period in 1982 show much smaller modulations, that Shemansky et al 1984 interpreted as indicating the outer heliospheric hydrogen density=0.11-0.12 cm-3 The second example, presented by Quemerais et al., 1996, used Voyager data from 1981-1993 At the end of that period, they found that the estimated solar Lymanalpha line-center flux modulation is damped by a factor of 0.4 (or smaller) in the Voyager (1 and 2) data, when the spacecraft were at distances of 56 and 44 A.U respectively They interpreted their data with a hot model for the hydrogen distribution and a Monte Carlo calculation for photon scattering, and concluded that the observed degree of damping was consistent with a hydrogen density of 0.15 +/- 0.10 cm -3 Their narrative suggests that they actually ruled out 0.05 cm -3 and 0.25 cm-3, suggesting acceptable hydrogen densities fell more in the range ~0.10-0.20 cm-3 The third example, previously unpublished, comes from examination of recent Voyager upwind data (looking generally upwind, ecliptic longitudes 258-270 degrees, ecliptic latitudes 15-25 degrees) from 2003-2004 obtained from 88.8-92.6 AU from the Sun and comparisons with simultaneous measurements of the upwind hemisphere from the downwind Cassini UVIS as it approached Saturn Figure shows the Cassini trajectory, while Figure shows a high signal-to-noise ratio UVIS Lyman-alpha spectrum The UVIS data spatial and temporal variations are in reasonable agreement with an optically thin model to be discussed below The waves in the Voyager data and the optically thin model also generally agree in phase and shape, but the waves in the data are damped by a factor of about 0.21 compared to model values produced in the optically thin model Optically thin model Hot models for hydrogen (e.g., Thomas 1978) begin with initial thermodynamic parameters for the neutral hydrogen at large distances from the sun (usually assumed to be near the termination shock) These key parameters are the neutral hydrogen density, temperature, and velocity For density we try 0.085 cm -3, to match the termination shock value from a full 2-shock "Model 2" provided by Izmodenov described in Table For hydrogen bulk velocity we use 20 km/s (Clarke et al., 1998), and a temperature of 12000 K based on estimates from SOHO SWAN H absorption cell data (Costa et al., 1999) The degree of damping cannot be estimated without a reliable model for solar activity We use the measured solar Lyman-alpha values provided by Tom Woods based on measurements from Earth orbit by SME (Solar Mesosphere Explorer), UARS SOLSTICE, SORCE (Solar Radiation and Climate Experiment) SOLSTICE, and TIMED SEE (Woods et al., 2005) These are line-integrated measurements of the output of the entire Sun Next, we estimate the amount of line-center Lyman-alpha radiation available to excite interplanetary gas based on work by Emerich et al., 2005 using the SOHO SUMER (Solar Heliospheric Observatory Solar Ultraviolet Measurements of Emitted Radiation) instrument They found the relationship between line-center and lineintegrated flux to be: f =0.64 F1.21 +/- 0.08 where f is the central solar spectral Lyman-α photon irradiance, expressed in units of 10 12 s-1 cm-2 nm-1 and F is the total Lyman-α photon irradiance, expressed in units of 1011 s-1 cm-2 This expression has the effect of varying the ratio of line center to line integrated flux from ~0.85 at solar minimum to 0.95 at solar maximum This expression also affects the derived solar Lyman-alpha radiation pressure used in determining the hydrogen atom trajectories in the hot model Our model also includes the time-dependence of two key loss processes for neutral hydrogen The largest loss process is charge- exchange with solar wind protons, producing fast hydrogen atoms unable to react to the solar line because of their large Doppler shifts Solar wind mass flux variability (Pryor et al., 2003) is included in the model using the OMNI database produced by the NSSDC The timedependence of a second major loss process, EUV photoionization of neutral hydrogen, is included using photoionization estimates taken from the Solar2000 Model (Tobiska et al., 2000, 2006) The amount of line-center Lyman-alpha seen at each longitude from Earth is used to infer the Lyman-alpha signal seen from the spacecraft in a line-of-sight integration through source regions at a variety of solar longitudes The hydrogen density model used to this is a modified hot model based on the work of Thomas 1978, and includes a variety of modifications discussed primarily in Pryor et al., 1992 to cope with latitude and longitude effects in Lyman-alpha We did not use our He 1083 nm technique (Pryor et al., 1996) for modeling Lyman-alpha data detailed variations in latitude as well as longitude because the National Solar Observatory He 1083 nm data sets are in transition to new instrumentation at this time The model needs to be slightly tuned to fit the spatial variations across the sky The major remaining free parameter to this is the "A" parameter that controls the solar latitude dependence of the charge-exchange lifetime of neutral hydrogen τsw The formula is (Witt et al., 1979): τsw(latitude)= τsw(equator)/(1 - A sin2(latitude)) Lyman alpha data from UVIS obtained during individual Cassini spacecraft rolls in 2004 (near solar minimum) indicate that a model A parameter value of 0.8 (Figure 4) fits the Cassini data better than an A parameter value of 0.0 (Figure 3) A=0 is appropriate for a spherically symmetric solar wind; A>0 is more appropriate for enhanced solar wind mass flux near the solar equator When applied to the Cassini UVIS time-series Lyman-alpha upwind data obtained from downwind near Saturn’s orbit in 2003-2004, the preferred model (using A=0.8) has time variations that track the Lyman-alpha data variations Data were obtained with the UVIS FUV detector in 24-25 s integration intervals Data were selected when the spacecraft was pointing in the upwind hemisphere within 30 degrees of the ecliptic plane Periods with obvious stars contaminating the data were removed Figure shows the rough agreement between data and a scaled model obtained for different instrument configurations: configuration 103 for occultation slit mode (8 mrad x 60 mrad), and configuration 104 for low-resolution slit mode (1.5 mrad x 60 mrad) Data (and model) were binned in time (by 40 24-s integrations for occultation mode, and by 96 24-s samples for low-resolution mode) to improve the signal-to-noise ratio The results are largely independent of slit width, as they should be for a diffuse source with wellunderstood detector backgrounds In some cases the model waves are the same size as the data waves We interpret the agreement between data and model to mean that for Cassini, the H column between the Sun, the relatively near-Sun scattering points that dominate the observed intensity, and the observer remains optically thin When the same single-scattering model is applied to the Voyager data from 2003-2004 (Figures 6, 7), damping is seen: the periodic waves in the data, while statistically significant, are much smaller than the waves in the optically thin model We estimate the damping factor from the data and model comparison as follows First, the data was scaled to the model, creating an empirical calibration factor Next, a leastsquares fit of a line to an 81-day running smooth of the model is subtracted from the model and the data to obtain detrended data and model, leaving the waves but no mean offset from Then, a least-squares fit of the detrended data to the detrended model was used to find the damping factor of 0.21 That is, the 27-day wave amplitude is about times smaller in the data than in the optically thin model The resulting fit of a "damped" model with waves reduced in amplitude by a factor of 0.21 to the original data is plotted in figure The conclusion is that as Voyager has traveled from 56 A.U to 88-92 A.U., the damping factor has dropped from ~0.4 (Quemerais et al., 1996) to the new value of ~0.2, a trend anticipated by those authors Interpretation of the damping factor in terms of a hydrogen density requires additional modeling with Monte Carlo techniques, to be discussed below Multiple scattering models Pryor, W R., C A Barth, C W Hord, A I F Stewart, K E Simmons, J J Gebben, W E McClintock, S Lineaweaver, J M Ajello, W K Tobiska, K L Naviaux, S J Edberg, O R White, and B R Sandel, Latitude variations in interplanetary Lyman-a data from the Galileo EUVS modeled with solar He 1083 nm images, Geophys Res Lett., 23, 1893-1896, 1996 Pryor, W R., J M Ajello, D J McComas, M Witte, and W K Tobiska, Hydrogen atom lifetimes in the three-dimensional heliosphere over the solar cycle, J Geophys Res 108, doi:10.1029/2003JA009878, 2003 Quemerais, E and J.-L Bertaux, radiative transfer in the interplanetary medium at Lyman alpha, Astron Astrophys 277, 283-301, 1993 Quemerais, E., J.-L Bertaux, B R Sandel, and R Lallement, A new measurement of the interplanetary hydrogen density with ALAE/ATLAS 1, Astron Astrophys 290, 941-955, 1994 Quemerais, E., B R Sandel, and G de Toma 26 day modulation of the sky background Lya brightness: estimating the interplanetary hydrogen density Astrophys J 463, 349358, 1996 Richardson, J D., Y Liu, C Wang, and D J McComas, Determining the LISM H density from the solar wind slowdown, Astron Astrophys., this issue, 2007 Scherer, H., Lyman alpha transport in the heliosphere based on an expansion into scattering hierarchies, Astron Astrophys., 309, 957-969, 1996 Scherer, H., Resonance Glow of the Neutral interplanetary gas, in The Outer Heliosphere: Beyond the Planets, K Scherer, H Fichtner, and E Marsch, eds., p 91-136, 2000 Shemansky, D E., D L Judge, J M Jesson, Pioneer 10 and Voyager observations of the interstellar medium in scattered emission of the He 584 and H Ly-a lines, in IAU Colloquium 81, Local Interstellar Medium, ed F C Bruhweiler, Y Kondo, and B D Savage (NASA CP-2345), p 24, 1984 Slavin, J D., and P C Frisch, The boundary conditions of the heliosphere: photoionization models constrained by interstellar and in situ data, Astron Astrophys., this issue, 2007 Stone, E., A C Cummings, F B McDonald, B C Heikkila, N Lal, and W R Webber, Voyager explores the termination shock region and the heliosheath beyond, Science, 309, 2017-2020, 2005 Thomas, G E., and R F Krassa, OGO-5 measurements of the Lyman α sky background, Astron Astrophys., 11, 218-233, 1971 Thomas, G E., The interstellar wind and its influence on the interplanetary environment, Ann Rev Earth Planet Sci., 6, 173-204, 1978 Tobiska, W K., T Woods, F Eparvier, R Viereck, L Floyd, D Bouwer, G Rottman, and O R White, The SOLAR2000 empirical solar irradiance model and forecast tool, J of Atm and Sol.-Terr Phys., 62, 1233-1250, 2000 Tobiska, W K., and S D Bouwer, New developments in SOLAR2000 for space research and operations, Adv Space Res 37, 347-358, 2006 Witt, N., J M Ajello, and P W Blum, Solar wind latitudinal variations deduced from Mariner 10 interplanetary H (1216 A) observations, Astron Astrophys 73, 272-281, 1979 Wood, B E., W R Alexander and J L Linsky, The properties of the local interstellar medium and the interaction of the stellar winds of epsilon Indi and lambda Andromedae with the interstellar environment, Astrophys J 470, 1157-1171, 1996 Wood, B E., S Redfield, J L Linsky, H.-R Muller, and G P Zank, Stellar Lya emission lines in the Hubble Space Telescope archive: intrinsic line fluxes and absorption from the heliosphere and astrospheres, Astrophys J Supp Ser 159, 118-140, 2005 Woods, T E., F G Eparvier, S M Bailey, P C Chamberlin, J Lean, G J Rottman, S C Solomon, W K Tobiska, and D L Woodraska, Solar EUV Experiment (SEE): Mission overview and first doi:10.1029/2004JA010765, 2005 results, J Geophys Res., 110, A01312, Figures Figure The Cassini spacecraft's trajectory to Saturn placed it generally downwind in 2003-2004 Downwind is indicated (TO DO) Figure Summed UVIS Lyman-alpha spectrum The line is spectrally unresolved by the instrument, but is measured with high signal-to-noise ratio Figure Top panel: Blowup of individual Cassini spacecraft rolls in 2004 showing the model shape for the charge-exchange lifetime parameter A=0.0 is not in good agreement with the UVIS Lyman-alpha data Middle panel: ecliptic longitude of the UVIS pointing as a function of time in 2004 Bottom panel: ecliptic latitude of the UVIS pointing as a function of time in 2004 Figure Blowup of individual Cassini spacecraft rolls showing the model shape for A=0.8 is in better agreement with the UVIS data Figure UVIS data modulations observed from downwind on Saturn approach in the period 2003.5-2004.5 are generally in good agreement with the optically thin model, except for a scaling factor Figure Voyager Lyman-alpha daily average count rates for the same period were obtained with the spacecraft upwind (88-92 A.U from the Sun, at ecliptic longitude, ecliptic latitude 34 degrees) looking in the upwind direction (ecliptic longitude 263 degrees, ecliptic latitude 20 degrees) A 3-day running smooth has been applied to the data Estimated statistical errors are 1-2% of the daily average values Statistically significant modulations due to the Sun's rotation are observed in this period Figure Comparing the Voyager data from this period to the optically thin model shows many of the same features are present in both time-series, but with greatly reduced amplitude in the data This indicates that multiple scattering is important at this distance from the Sun Figure Improved agreement between the model and the Voyager data from late 2003 to early 2004 is obtained in this case by reducing the amplitude of the 27-day waves in the optically thin model by a damping factor of 0.21 Figure Top panel: Monte Carlo simulations of the Lyman-alpha intensity upwind viewed from the upwind axis as a function of heliocentric distance using Model for extreme cases: a bright spot on the Sun on the upwind axis, and a bright spot on the Sun on the downwind axis Calculations were performed at 10 A.U steps; displayed values are a 3-point running average to minimize statistical fluctuations Middle panel: The expected "27-day" upwind modulation in the Lyman-alpha brightness as seen from the upwind axis as a function of heliocentric distance is shown for Model and Model It is computed from the extreme cases explained in the top panel Bottom panel: The damping factor upwind (modulation divided by the modulation near the Sun) as seen from the upwind axis as a function of heliocentric distance ... smooth of the model is subtracted from the model and the data to obtain detrended data and model, leaving the waves but no mean offset from Then, a least-squares fit of the detrended data to... estimates of ~0.16 and 0.11 cm-3 respectively Quemerais et al., 1996 studied Voyager and Lyman-alpha data from 1981-1993 and found the solar waves were damped in amplitude by a factor of ~0.4 at... 1984, was Voyager and Pioneer 10 interplanetary Lyman-alpha modulation data from 1982 used to infer hot model hydrogen density at large distances from the sun Voyager interplanetary data obtained